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1

OS Structure,

Processes & Process Management

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Recap

OS functions Coordinator

Protection Communication Resource management

Service provider File system, device handler,

Questions: Ho can the OS perform these functions! Ho is an OS invo"ed! #hat is the structure of the OS!

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3

An Operating System in Action

CP$ loads %oot program from RO& 'e(g( )*OS in PC+s

)oot program: -.amines/chec"s machine configuration 'num%er of CP$+s, ho

much memory, num%er 0 type of hardare devices, etc( )uilds a configuration structure descri%ing the hardare 1oads the operating system, and gives it the configuration

structure

Operating system initiali2ation: *nitiali2e "ernel data structures

*nitiali2e the state of all hardare devices Creates a num%er of processes to start operation 'e(g( getty in

$3*4, the #indoing system in 35, e(g(

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O.S. in Action (Contd)

6fter %asic processes have started, the OS runs user

programs, if availa%le, otherise enters the idle loop

*n the idle loop: OS e.ecutes an infinite loop '$3*4 OS performs some system management 0 profiling OS halts the processor and enter in lo7poer mode

'note%oo"s

OS a"es up on: *nterrupts from hardare devices -.ceptions from user programs System calls from user programs

5o modes of e.ecution $ser mode: Restricted e.ecution mode 'applications Supervisor mode: $nrestricted access to everything 'OS

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Control Flow in an OS

Operating System &odules

*dle1oop

From %oot

*nitiali2ation

R5*

*nterruptSystem call

main'

-.ception

Supervisor &ode

Return touser mode

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On Interrupts

Hardare calls the operating system at a pre7specifiedlocationOperating system saves state of the user programOperating system identifies the device and cause ofinterrupt

Responds to the interruptOperating system restores state of the user program 'ifapplica%le or some other user program-.ecute an R5* instruction to return to the user program$ser program continues e.actly at the same point it as

interrupted(

8ey Fact: 3one of this is visi%le to the user program

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On Exceptions

Hardare calls the operating system at a pre7specifiedlocationOperating system identifies the cause of the e.ception 'e(g(divide %y 9*f user program has e.ception handling specified, then OS

adust the user program state so that it calls its handler-.ecute an R5* instruction to return to the user program*f user program did not have a specified handler, then OS"ills it and runs some other user program, as availa%le

8ey Fact: -ffects of e.ceptions are visi%le to user programsand cause a%normal e.ecution flo

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On System Calls

$ser program e.ecutes a trap instruction 'system callHardare calls the operating system at a pre7specifiedlocationOperating system identifies the re;uired service andparameters 'e(g( open'filename, O

Operating system e.ecutes the re;uired serviceOperating system sets a register to contain the result ofcall-.ecute an R5* instruction to return to the user program$ser program receives the result and continues

8ey Fact: 5o the user program, it appears as a function calle.ecuted under program control

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Operating System

(process/device/memory management,

file systems, interprocess communication, )

Operating System

(process/device/memory management,

file systems, interprocess communication, )

Operating System oday!ig"#le$el so%tware arc"itecture

MemoryMemory

Instruction Execution & Interrupt ProcessingInstruction Execution & Interrupt Processing

User ApplicationsUser Applications

Window

System

Window

SystemCommand

Interpreter

Command

Interpreter

I/O DevicesI/O Devices

MiddlewareMiddleware

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Operating System Structures

&onolithic OS 'e(g(, $ni. &icro7"ernel OS 'e(g(,&ach, -.o"ernel,

Memory ManagementMemory Management

CPU SchedulingCPU Scheduling

Process ManagementProcess Management

HardwareHardware

Network SupportNetwork Support

SecuritySecurity

File SystemFile System

Command InterpreterCommand Interpreter

Device ManagementDevice Management

NetworkSupport

NetworkSupport

MemoryMgmt.

MemoryMgmt.

WindowServer

WindowServer

FileServerFile

Server

......

HardwareHardware

CPUScheduling

CPUScheduling

Device

Drivers

Device

DriversInterrupt

Handler

Interrupt

HandlerBoot and

Init.

Boot and

Init.

Message PassingMessage Passing

APIAPI

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Summary

6n OS is ust a program: *t has a main' function, hich gets called only once 'during

%oot 1i"e any program, it consumes resources 'such as memory, can

do silly things 'li"e generating an e.ception, etc(

)ut it is a very strange program: *t is ?entered@ from different locations in response to e.ternal

events *t does not have a single thread of control, it can %e invo"ed

simultaneously %y to different events 'e(g( system call 0 aninterrupt

*t is not supposed to terminate *t can e.ecute any instruction in the machine